LONDON (Reuters) - The campaign to keep Britain in the European Union has taken an 18-percentage point lead over the Out campaign ahead of a June 23 referendum, a telephone opinion poll by Ipsos MORI showed on Wednesday.

Ipsos MORI found 55 percent of those surveyed supported staying in the EU, a level the Evening Standard newspaper said was the highest in three months, while 37 percent wanted to leave and 8 percent were still undecided.

Sterling climbed to a two-and-a-half week high against the euro and a day's high against the dollar after the poll was published, while odds on gambling website Betfair priced in a 76 percent chance of a "Remain" vote, up from 73 percent on Monday.

Ipsos MORI's head of political research, Gideon Skinner, said some supporters of Prime Minister David Cameron's ruling Conservative Party who had previously opposed membership had switched to supporting the In campaign.

"Remain has been boosted by a Conservative swing, but they are also more likely to change their mind, so in this volatile election, with voters divided over the short and long-term impacts of their decision, nothing can be taken for granted," Skinner said in a statement.

Ipsos MORI conducted interviews with 1,002 adults between May 14 and May 16.

Earlier on Tuesday The Times newspaper reported an online poll by YouGov showing 44 percent of people wanted to stay in the EU and 40 percent wanted to leave.

(Reporting by David Milliken, editing by Estelle Shirbon and Guy Faulconbridge)